In modern mobile communications systems a mobile terminal such as a mobile phone, is free to move and connect from one cell to another within the mobile communication system. If a mobile terminal does not have an ongoing call, the entering in a new cell only results in registering to the new cell. If a mobile terminal is handling a call during entering the new cell, the call must also be switched to the new cell by a way which causes as little disturbance to the call as possible. The process of cell change during an ongoing call is called a handover, which implies a change of serving cell. To be connected to the best cell is crucial for both capacity and cell edge bit rate. Because of the varying quality of the radio environment the best cell can change rather frequently. Median best cell duration is can be in the range 0.5-4 seconds. Much shorter durations are common at the cell edge.
To be able to detect a need for a handover and to select a suitable destination cell for the handover, various kinds of measurements are required to determine the connection quality and field strength levels of the neighbouring cells. The handover in turn involves measurement and signalling procedures, which procedures are very time consuming. The time between a new best cell is detected and the mobile terminal is connected to that cell is here denoted the execution time. To improve coverage, capacity and quality, it is of interest to minimize the execution time.
A typical handover procedure can be divided into a preparation phase and an execution phase. The basis for handover is measurements by the mobile terminal on signals from the cells which is performed in the preparation phase. A mobility controller in the network orders the mobile terminal to measure on a defined set of reference signals that are configured as neighbor cells. Typically around 20 neighboring cells are configured to cover handover possibilities needed for coverage reasons in the whole cell area. There are also self configuration algorithms that are based on mobile measurements. The terminal needs to assign resources to listen to reference signals in e.g. Code Division Multiple Access (CDMA). In Global System for Mobile communication (GSM), Time Division Multiple Access (TDMA) and Personal Digital Cellular (PDC) the measurements are sent periodically from the mobile terminals to the network that filters and applies a hysteresis, so that a handover is only performed if the new best cell is at least a certain margin better than the current cell. In Wideband CDMA (WCDMA) the mobile terminal performs filtering and hysteresis and sends a triggering event to the network. The hysteresis and filters are typically, e.g. in GSM and WCDMA, applied to avoid too frequent handovers. The execution phase then starts, during which execution phase the data transfer is interrupted. During this interruption and execution phase the network sends a handover command to the mobile terminal, the command comprises information about which the destination cell is and which destination base station to communicate with selected among candidates. The mobile terminal then listens to the selected destination base station which destination base station broadcasts system information. System information is in this document defined as resource assignment for accessing the destination cell. This may take some time since the mobile terminal has to wait for the system information to be broadcasted. Based on the obtained system information, random access can be performed. Random access is “contention-based”; i.e. the mobile terminal transmits on a channel being common for many mobile terminals wishing to enter the destination cell. Since more than one mobile terminal is competing about resources/channels, collision may appear which may lead to unsuccessful access and that the mobile terminal must try again. Further in the execution phase and during the interruption of data transmission, measurements of timing alignment and power that mobile terminal shall use for transmission when communicating with the destination base station after the handover, are performed by the destination base station and the value of the timing alignment and required power is sent from the destination base station to the mobile terminal. The value of timing alignment may e.g. be defined by the destination base station by measuring mobile terminal transmission signals and decide how much the mobile terminal has to adjust its point of time of transmission to synchronize to the point of time when the destination base station requires to receive the mobile terminal transmission. Further in the execution phase and during the interruption of data transmission, a scheduling request is sent from the mobile terminal to the destination base station and a scheduling grant is sent from the destination base station to the mobile terminal. Thereafter the handover execution is finished and the transfer of data can continue. The execution phase is very time consuming which makes the interruption of data transmission long. This in turn may lead to reduced end user perception. Furthermore, a slow handover execution gives a poor cell edge bit rate.
To improve the handover not only downlink measurements may be used. In TDMA and PDC, an uplink measurement on a handover candidate is performed after triggered but before execution to verify the downlink measurement. Even though this solution improves the time of handover execution, the process is still quite time consuming.